The need to effectively control solidification of a metal melt in a mold to make castings with acceptable practical service properties forces researchers and engineers to look constantly for new approaches in order to radically improve the quality of castings because castings acquire their basic service properties at the crystalline structure solidification stage.
Until recently, all methods for controlling processes developing during solidification of metal melts have been confined to influencing the thermal processes within the melt and at the heat exchange boundary. In this environment, the two-phase solidification front zone forming at the casting periphery obstructs removal of latent heat as it moves toward the center at a slowing rate, causing variation in the grain size and raising pressure within the melt as the contracting solid phase grows, and in this way provoking the release of dissolved gases into the melt. This organization of the solidification process is ineffective, to an extent, and results, no matter what option is used, in the casting grain size developing a gradient and hence anisotropy of properties. Moreover, when solidification is effected by heat removal, defects such as micro- and macro-voids and various forms of liquation cannot be avoided. Attempts are made to offset the structural defects of castings made by an existing method in which the melt is solidified from the periphery to the center thereof. A good example of this is a method in which a fine structure is produced by activating the melt with various impurities, mostly those having a higher melting point, the particles thereof serving as solidification centers. It is appropriate to interpret the mechanism in which solidification centers are formed as operation of “micro-refrigerators.” More refractory inclusions have a stable crystalline structure at solidification temperatures of the host metal, and their atoms are able to “take away” some of the energy from the melt components in localized zones of the melt. This “take-away” energy creates conditions favorable enough to begin solidification in these zones.
A similar solidification mechanism develops when various alloys are used to “multiply” their structure within the melt, a process now known as “heredity.” Whatever the method used to produce alloys, they have a structure fragmented considerably and have a slightly higher melting point than the host metal because of large contact surface areas of their components. Accordingly, dissolution of a partially molten alloy in the host metal, if slightly overheated, results in more solidification centers developing as in the example described above. Use of alloys, as also addition of a modifier to complete volume solidification to produce a fragmented structure gives rise to several problems. Production of a desired structure is influenced significantly by various parameters such as temperature, dissolution quality, distribution of alloy components over the melt volume, and a few other factors. Many research projects are centered on these problems. Also, excessive pressure is produced within the melt, for example, in a thermostatic gas chamber. In this example, interatomic distances are reduced, and interaction energy rises. Since, however, excessive pressure is built in all examples within the entire volume of the melt, and heat is removed, as it has been before, from the surface, the solidification front is directed from the periphery to the center, causing all possible casting defects typical of prior art methods. The only advantage to be gained from this method is possibly improved mold filling and an insignificant improvement in casting structure uniformity.
An analysis of defects developing during solidification suggests a conclusion that they ultimately result from the method in which solidification is conducted by removing heat from the casting surface.
Indeed, the solid peripheral phase, like the solidification front as well, shuts off the accompanying gas phase inside, contributing to blistering, cracking, liquation, and so on.
A method is, however, known in the art to be used for making castings by directed solidification of the melt (U.S. Pat. No. 1,424,952), wherein a casting is formed in a nonuniform field of force of a rotating mold as the melt volume is cooled in its entirety (rather than in a selected direction). The mold rotation speed is chosen in this case so as to expose the melt to a pressure required to overcool the melt to the extent equal to the interval of its metastability. In these conditions, undirected cooling of the melt causes solidification thereof to be directed from the periphery toward the rotation axis of the mold. This effect is achieved by the solidification temperature rising under the influence of pressure built up in the peripheral zones of the melt, being higher than pressure in zones closer to the rotation axis of the mold.
To put this method into practice, however, a high pressure is to be built up with the possibility of the casting mold containing the melt being broken.
Moreover, the constant rotation speed of the mold to produce the desired pressure results in anisotropy of the casting structure and strength characteristics because the solidification front shifts as overcooling decreases continuously toward the rotation axis of the mold.
Accordingly, the conclusion that can be drawn from the above is that a localized elevated pressure zone produced in the casting volume could allow solidification to be controlled effectively from that zone toward the casting periphery. A solidification front moving from that zone toward the periphery could allow gas pockets and unbound intermetallic compounds to be pushed out to the casting surface, prevent development of shrinkage cracks, blisters, and so on.